The invention concerns generally the transmission of information between a terminal of a cellular radio system arid an accessory device. Especially the invention concerns an accessory interface that sets relatively few structural requirements to the terminal.
A terminal of a cellular radio system has conventionally been a mobile telephone, but together with the evolution of the cellular radio systems a number of other terminal types have emerged. According to expectations, the terminal of a future cellular radio system will combine at least some of the features of a video telephone, portable telefax, palmtop computer, positioning device and many other portable electronic devices previously known as separate units.
Already at the mobile telephone stage a need has been recognised to furnish the terminal with an accessory interface, and this trend will certainly continue intensively with the more diversifed terminal types. An accessory device is typically an auxiliary computer, a camera, a printer, a mass memory device or some other device that has some features that are missing from the mobile terminal and that is meant to be used locally, i.e. in the immediate vicinity of the terminal, in the same room or at least within a short walking distance. The conventional way of providing an interface for transmitting information between an accessory device and a terminal is a cable. Recently also an infrared interface has gained popularity.
The drawbacks of the prior art accessory interfaces relate to the structural requirements they set to the terminal. A cable necessitates a cable connector to be built in the terminal, and an infrared interface requires an infrared transceiver. These must be located in the terminal in suitable positions for easy operation, whereby they reserve a part of the outer cover of the terminal. A cable connector is prone to dirt and corrosion and its appearance is not always in accordance with the esthetical aims of a designer. The lens of an infrared transmitted may become dirty or scratched, causing unreliable transmissions. Additionally one must take into account the driver circuits that are indispensible for operating a cable connection or an infrared transceiver but require space from within the terminal and consume power.
It is an object of the present invention to provide an accessory interface for the transmission of information between a terminal and an accessory device without the drawbacks that are characteristic to the prior art solutions. It is a further object of the invention to provide an accessory interface that sets only few structural requirements to the terminal. Additionally it is an object of the invention to provide a method for using said accessory interface.
The objects of the invention are achieved by using at least partly the same structural elements for the accessory interface that are used for the radio interface between the terminal and the other parts of the cellular radio system.
It is characteristic to a terminal of a cellular radio system according to the present invention that the terminal comprises means for identifying a radio channel on which there is no perceivable radio traffic in the cellular radio system and means for establishing a local communication connection between the terminal and an accessory device on the identified radio channel.
The invention applies also to an accessory device with the characteristic feature that it comprises an adjustable radio transceiver for establishing communication connections with a terminal of a cellular radio system on a selectable radio channel.
Additionally the invention also applies to a method comprising as its characteristic features the steps of
identifying a radio channel on which there is no perceivable radio traffic in the cellular radio system, and
establishing a communication connection between the terminal and the accessory device on the identified radio channel.
According to the invention, one should not neglect the fact that by definition, any terminal of a cellular radio system already includes an interface: the radio interface between the terminal and a base station that belongs to the fixed parts of the cellular radio system. By using basically the same or similar structural parts also for the accessory interface it is possible to achieve major savings in the component count, physical size and power consumption of a terminal.
If some of the functions and components related to the radio interface must be shared between the actual radio interface and the accessory interface, a suitable multi-use or multiple access scheme must be applied. Here again we may note that in the cellular radio system there already exists a multiple access scheme for allowing a large number of terminals to use the services of a relatively limited number of base stations. Conventional multiple access schemes comprise TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) and FDMA (Frequency Division Multiple Access) arrangements as well as combinations of these. According to an advantageous embodiment of the invention the aspects of the cellular radio system""s multiple access scheme are applied to divide the capacity of the terminal between the actual radio interface and the accessory interface.
In a CDMA embodiment of the present invention the accessory interface of the terminal takes advantage of the fact that in any given cell there is always only a very limited number of spreading codes in use compared to the total available code space, i.e. the number of spreading codes that may be generated according to the existing code generation rules. One of the unused spreading codes is simply selected for use in the communication between the terminal and the accessory device. Alternatively a separate code space may be used for the purposes of the accessory interface.
In a TDMA embodiment of the present invention there may be unused time slots, during which the terminal may communicate with the accessory device. Similarly in a FDMA embodiment some otherwise unused frequency is reserved for communication between the terminal and the accessory device. However, these embodiments of the invention are not regarded as being as advantageous as the CDMA embodiment, because the number of available time slots and the number of available frequencies is usually much more limited than the number of available spreading codes. Also the structure of the radio frequency part of the terminal would have to be slightly more complicated.